Patch #936774: unify the display of data descriptors, including slots,

Patch #936774: unify the display of data descriptors, including slots,
properties, and custom descriptors.

* removed special handling of properties

* added special handling of data descriptors - All data descriptors are grouped
  together in a section. For each item, the attribute name and doc string, if
  present, is displayed.

* disabled display of __slots__ attribute - since slots are descriptors, they
  are listed in the section described above

Thanks to John Belmonte for the patch!